Large inflatable devices and fabric structures, such as airship hulls, radomes, aerostats, tension roofs, all require large pieces of fabric, or composite materials including fabric layers, to be joined together, since a these structures cannot typically be formed from a single piece of fabric. The joints between the fabric sections typically must be gas-tight, and are preferred to be as light in weight as possible, while being durable enough to withstand service conditions for at least the intended life span of the structure, e.g., ten years. The joints are also preferably capable of being quickly repaired using reasonably inexpensive and uncomplicated equipment. While service loads on airship hull joints and other fabric structures can very widely depending upon the type and size, the service loads can be up to 400-500 lbs/inch at the joint, and the joint may be exposed to temperatures of up to solar driven thermal conditions of 140° F. for a prolonged duration.
Since the joints are required to be gas tight, liquid tight, avoid stress concentrations, and be repairable, the use of mechanical fasteners such as sewing or rivets are not desirable for joining the sections. Heat sealing is a more useful and commonly used means for joining fabric sections together.
Thermoplastics, for example, have been used with success in some small airship applications to join hull sections together. In these lower stress applications, a non-cross linked thermoplastic, such as urethane, is applied between two fabrics, and the fabrics are joined together by heating the thermoplastic adhesive to a temperature of about 330° F. and applying a pressure of about 35 psi to the joint. The heat and pressure are applied to the joint for a relatively shore amount of time, typically less than about approximately one minute. This process can produce acceptable joints that can withstand service loads of up to about 100 lbs/inch at the joint. To produce acceptable joints under these conditions the following conditions must be observed:                1. The thermoplastic resin to fiber ratio must be nearly 1:1 by weight.        2. The thermoplastic resin must continuous coat the surfaces to be jointed.        
However, higher stress applications such as that present in heavy cargo-carrying airships, large raydomes, and super light inflatables all experience much larger service loads, e.g., 150-500 lbs/inch at hull joints, and thus require the use of higher strength fabric, for example fabrics including fibers having a tenacity of 10 grams/denier of higher, and correspondingly high strength hull joints. The Applicant has confirmed by testing that conventional joint forming techniques are not suitable for such high stress applications and prolonged exposure to temperatures of approximately 140° F., which is a standard requirement for airships and other fabric structures. Creep resistance in the joint is critical as the higher loads must be supported through the thermoplastic interface. Heat seal adhesives cannot have a high cross link density. As a class of material, all thermoplastics exhibit creep behavior to some extent. In the case of thermoplastic urethanes (TPU) the hard segment behavior of these polymers gives them some intrinsic creep resistance. However even TPU materials creep and the joint design must minimize this behavior.